[GMailUserImporter] [Error][user.name] Unexpected exception when processing user. Unexpected exception processing import. Message is: Google.Apis.Requests.RequestError
File not found: xxxxxxxxxx 
Message[File not found: xxxxxxxxxx] Location[file - other] Reason[notFound] Domain[global]
My Google Drive to Google Drive migration is resulting in failed users with 404 "file not found" errors
This error occurs when the migrating user is making a request on a file or folder that it doesn't have access to in the destination Google Drive environment. In almost all cases this occurs when all users and groups (and all group memberships) have not been created prior to Drive migration, and thus the required permission is not present on the migrated item.
Before you start a migration of this kind, you will encounter a prompt that explains this:
It is highly recommended to utilize Drive Readiness which will help you to ensure the prerequisites are met prior to migration, thus, preventing possible 404s.
You should always provision all of your users and groups before performing a Drive migration. This is required to ensure all sharing and Drive hierarchies are preserved correctly during the migration. If you are renaming users as part of the migration, you must make sure all import and export names are present and have been updated to the names in the new system. You should ensure you have provisioned all of your users before migrating your groups, and all addresses are mapped in the address replacements CSV.
FAQ: Why does it fail the migration for the user rather than skip and move on?
The reason that CloudMigrator fails the migration for the user is because it will more than likely break the integrity of the rest of the migration for that user (and any other user that migrates the same item(s)) and/or create other unwanted issues if it continues.